The immunosuppressants cyclosporin A (CsA), FK506 and rapamycin, have been shown to have disparate and often deleterious effects on bone morphology and bone mineral metabolism in rats in vivo. In the clinical setting these drugs are increasingly recognized as a significant etiological factor in the evolution of post-transplantation osteoporosis, with its attendant morbidity and mortality. The studies proposed in this application focus primarily on defining the in vivo mechanism(s) that underlie the bone loss associated with the use of immunosuppressant agents. All of the proposed studies make use of an established rat model. Serial measurements of serum calciotropic hormones and markers of bone remodeling will be performed during the course of each study. These biochemical indices of bone mineral metabolism will be complemented by sequential measurements of skeletal bone mineral density (BND), and detailed histomorphometric analysis performed on the rats proximal tibias at the completion of each 28 day experiment. These studies will include a comparison of the effects of CsA on bone mineral metabolism in nude (athymic) and immunocompetent rats. This study will help assess whether CsA acts directly on bone and bone marrow or via the immune system. A second study will utilize bones and bone marrow harvested from the above- mentioned animals to assess the role of bone cytokines/local factors in normal and CsA induced bone remodeling. RNA extracted from these bones and bone marrow will be probed, so as to measure ex vivo gene expression of the relevant cytokines and cyclophilins. A third study will examine the role of endothelin in CsA induced bone loss. The effects of CsA, FK506, and rapamycin on bone mineral metabolism will be ascertained using our rat model with concurrent ex vivo analysis of 1,25(OH)2D3 receptor and BGP gene expression in bone and bone marrow. A secondary focus is to ascertain whether certain experimental immune- modulation regimens are bone sparing. This will include and in vivo study examining the effects on bone of low dose cyclosporin G and rapamycin, compared to the effects of normal doses of either agent alone, as well as the effects of TGF-beta and interferon-y on CsA osteopenia. The information gleaned from the above studies will be of enormous value to the field of post-transplantation bone disease, and facilitate the development of rational preventative and therapeutic strategies.